Vermicular expanded graphite is a low bulk density, (usually between 0.002 and 0.02 gram per cubic centimeter), particulate, worm-like form of graphite. It is prepared by treating natural flake graphite with an intercalating agent such as fuming nitric acid, fuming sulfuric acid, mixtures of concentrated nitric and sulfuric acid, and the like. The treated graphite is then heated to a high temperature, e.g., above 500.degree. C., to expand the natural flake graphite to the light weight vermicular form. The preparation of the vermicular expanded graphite is well known in the art, and is described in greater detail, for example, in U.S. Pat. Nos. 3,389,964 and 3,323,869, which are incorporated herein by reference.
Vermicular expanded graphite is often compressed into various shapes and forms or onto various substrates or supports. The resulting compressed graphite structure has many desirable properties including, for example, high electrical and thermal conductivity, high anisotropic ratios, resistance to high temperature oxidation, and excellent mechanical properties. Because of these properties, compressed forms of vermicular expanded graphite have found utility as coatings for various metal substrates, for example, as described in U.S. Pat. No. 3,492,197.
A disadvantage to the use of compressed forms of vermicular expanded graphite as substrate coatings is that such graphite is subject to permeation by corrosive fluids upon prolonged contact.
To render natural flake graphite impermeable to corrosive fluids, it has been suggested to impregnate such graphite with a polymeric substance, such as, for example, an aqueous polytetrafluoroethylene dispersion. (See, for example, British Pat. No. 1,257.010.) However, low fluid permeability is not obtained by such impregnation even after repeated treatment due to the high water content of the dispersion.
Vermicular expanded forms of graphite have also been impregnated with certain polymeric substances. For example, U.S. Pat. No. 3,492,197 describes the blending of finely powdered inorganic or organic resins, such as polytetrafluoroethylene, with vermicular expanded graphite to improve the thermal, electrical, and mechanical properties of the expanded graphite. However, such treatment has not been found to impart to the graphite the desired impermeability to corrosive fluids, since powdered resins tend to agglomerate in localized areas of the graphite leaving other areas of the graphites open to fluid permeation.
U.S. Pat. No. 3,492,197 also describes the blending of a solution of inorganic or organic resins dissolved in a solvent with vermicular expanded graphite. However, such treatment is not desirable because many corrosion resistant resins, such as polytetrafluoroethylene, are insoluble in any solvent. Moreover, the use of a solvent can cause particularly poor compression characteristics in expanded graphite due to the tendency of vermicular expanded graphite to retain a high residual solvent content.
A need therefore exists for an improved method of forming vermicular expanded graphite material which is impermeable to fluids, especially corrosive fluids at high temperatures.